Nothing causes engine failures more than an incorrect air fuel ratio. We show you why it happens and how to prevent it.
The air fuel ratio on cars isn’t sexiest subject in tuning, but it can be the difference between life and death for your engine. What’s scientifically best, what’s best for fuel economy, what’s best for power, and what’s best for your engine’s life are four very different things.
And to complicate things further, they vary from engine to engine, and even fuel to fuel. The reality of it though is you that don’t need to be an expert, that’s the job of your tuner.
But, understanding the basics of air fuel ratios could well save your engine, and save you from a very expensive repair bill.
Air fuel ratio guide
When parts break on your tuned car, it’s rarely to do with them simply not being strong enough. More power and revs push things far closer to their limits, but if your head gasket has blown or your piston has melted, it’s highly likely this was due to a detonation caused by less than optimal air fuel ratios.
All too often people fall into the trap of thinking, “Oh I’ve blown a head gasket, I’ll get a stronger one to stop it happening again.” But often the reliability issues continue, because it’s detonation that’s causing it to fail rather than the components being weak. The vast majority of cars can take around double the standard power on standard internals if the air/fuel ratio is correct. But if it’s not, even the strongest engine will fail quickly at relatively low power.
So, why can bad air fuel ratio kill my engine?
One word, detonation. Detonation is where part or all of the air/fuel mix ignites on its own – without the aid of the spark plug. This happens most commonly when lean mixtures increase combustion chamber temperature to a level that the fuel spontaneously combusts.
While in mild form this is fairly harmless, the severe ‘det’ common when tuning cars is highly destructive. Detonation causes enormous cylinder pressures, far in excess of even what the highest boost pressures could give without detonation, and this causes – at the very least – your head gasket to blow, and often melts your pistons or more, ending your engine’s life.
This catastrophic failure can happen very quickly, and this is why head gaskets are often designed to be the weakest point in an engine – it’s a lot cheaper to fix the head gasket than to carry out a full bottom end rebuild!
The science behind air fuel ratio
Air fuel ratio (AFR) is, unsurprisingly, the ratio of air to fuel injected into your engine. The air is either sucked (normally aspirated), blown (turbo or supercharged), or injected (nitrous) in, while the fuel is injected in, or sucked in the case of carburettors. The ratio of this mixture affects your fuel economy, performance, reliability, or even if your car runs at all.
The scientifically “perfect” mixture for normal petrol is 14.7:1, that is 14.7 times the mass of air to fuel, but that doesn’t mean a car should run at that – far from it. For optimum fuel economy 16-17:1 is usually best, leaner than that and the car will begin to misfire. Maximum power is usually found between 12-14:1, but this may be too lean for safety on many engines. For maximum reliability at full power, air fuel ratios from 10.5-12.5:1 are considered best, depending on the engine. Richer than around 10.5:1, you start to get noticeable black smoke from the exhaust and the car can struggle to run properly without misfiring.
Tuning complicates things further, as varying compression ratios, camshafts, boost pressures, and so on all affect the ideal air fuel ratio needed at any one point, indicating the importance of having your car remapped as you add tuning components to it. As I’m sure you’ve gathered by now, to get a good overall car you need to have a mixture of these AFRs, depending on how the car is being driven at any moment in time, and thankfully that’s what modern fuel injection can supply you, provided the car has been mapped properly…
How do I tell if my air fuel ratio is safe?
While you can buy air fuel ratio metering equipment, which is useful for a seriously tuned car or a competent home tuner, the best way of knowing your AFR is safe is to go to a trusted and reliable tuner to get your tuning work done. What is safe on one engine isn’t necessarily on another, so enlisting the help of an expert to make sure things are safe is the most important thing here. It’s also well worth getting things checked over every six months or so to make sure everything is still how it should be.
If you’re convinced you need an AFR meter, make sure it’s a wide band setup, rather than the cheaper narrow band setup. The reason for this is because a narrow band only tells you three things, whether it’s at 14.7:1, leaner than it, or richer than it. Considering in performance terms that even 14.7:1 is far too lean for maximum safe performance, a narrow band sensor is useless.
My air fuel ratio is fine, does that mean my engine won’t blow up?
You can never, even with the most well set up engine in the world, be totally sure an engine won’t die. But, as long as your ignition timing isn’t wildly wrong and the lump is in good general health, having a safe AFR is generally the main key to it having a long life under hard use.
Other issues, oil, cooling, and general strength of components can always destroy an engine, but the number one killer, especially among tuned turbo engines, is detonation due to incorrect air fuel ratios.
What will change my air fuel ratio?
Electronics fail on cars from time to time, that’s just a fact of car ownership, but sometimes a sensor failure can have catastrophic effects on your engine. There are various sensors that affect fuelling, but air flow, air pressure, and temperature sensors are the most likely to drastically affect your fuelling if they partially or completely fail. Failing in a manner that makes the car run rich means you should notice a problem but no damage will be done, but failing and making the car run lean can end in disaster.
There is little you can do to stop this happening aside from sorting out any running issues the minute you notice them, but getting your AFR checked every six months could indicate early signs of a problem. Changing sensors for upgraded versions, especially things like airflow and pressure sensors, will also cause lean running problems if the car hasn’t been remapped to suit, so be careful what order you upgrade components.
Changing to higher lift and longer duration cams lets your engine breathe more easily and this often means big fuelling changes are needed not only to keep the engine safe, but to extract maximum performance from the cam. At high rpm, where the improved breathing gives noticeable power improvements, extra fuel is usually needed to prevent the car from running lean. But at low rpm, especially around idle, a richer mixture is often needed to keep drivability, as longer duration cams mean more fuel is wasted out of the exhaust at low rpm.
Fuel system problems
All components have their limits of capability, and when tuning engines you often hit the limit of what your fuel injectors, fuel pump, and even fuel lines and filter can cope with. Beyond their maximum flow rates your engine will run lean and eventually fail. Age is another big factor with fuel systems, as they gradually get clogged up with dirt, and fuel pump wiring can also deteriorate, all of which will reduce flow. The final point worth mentioning is the vacuum pipe to the fuel pressure regulator, especially on turbo and supercharged engines. If this pipe splits or is removed, the engine will run incredibly lean on boost, and sometimes won’t survive a single full throttle run.
Fuel system changes
Fitting improved fuel system parts is vital on most cars tuned beyond the basic Stage 1 parts, but fitting them without the car being set up to suit can cause big problems. Installing bigger injectors without a suitable remap can cause the engine to massively over-fuel, which, while not causing instant death, isn’t good for economy or performance. The most potentially dangerous upgrade is the fuel pressure regulator, as you can be almost assured an upgraded replacement won’t be set for your correct pressure, and while too much pressure is not a killer, too little equals lean running and a dead engine. Because of this, a fuel pressure gauge is a vital tool when replacing the regulator.
You may consider an exhaust to be a minor upgrade, but if it gives significantly more flow and performance, the car will need more fuel to stop the engine running dangerously lean. Some engines automatically compensate to some extent when making changes like this, but conversely, many cars – turbocharged ones in particular – can run dangerously lean from a simple full exhaust system swap, and therefore need a matching ECU remap to suit.
Cooler air may decrease the chances of detonation, but it’s also much denser and therefore feeds the engine with more oxygen, so the engine would require more fuel not to run lean. Just like the exhaust, some cars will compensate for this and cause no problems, but some cars need a remap to compensate for the intercooler, and it’s well worth making sure.
Many people don’t realise this, but even if your boost pressure is exactly the same, if you fit a larger turbo you will be feeding your engine with a lot more air, meaning it can just as easily run lean and die as it would with a boost increase. The reason for this is a larger turbo not only gives cooler, denser air, but also lets a lot more air out the exhaust, which in turn lets more air in the cylinders. Without the extra fuel to compensate, yes, you guessed it, it will run lean, detonate, and the engine will fail.
Boost pressure increase
This is the biggest cause of lean air/fuel ratios, and why many turbo cars have undeserved reputations for blowing head gaskets and melting pistons. Boost increases improve power and torque due to the subsequent large airflow increases, but without the corresponding increase in fuel the car will run dangerously lean and detonate. It’s not just the increased airflow that causes problems either, more boost pressure means higher air temperatures, and with higher temps come an increase in the chances of detonation, making it a double edged sword.
Detonation is the main reason to worry about your air fuel ratio, but that is because normal pump fuels aren’t very resistant to it. While super unleaded is widely available at most pumps, its octane rating of 97-99 RON is fairly low, and a big part of the reason we have to run so rich, especially on turbo cars, to keep things safe.
Expensive and not available in your local petrol station, race fuel is generally reserved for track-only cars, and would need a remap to get the most out of it if you put it in your vehicle. It can be incredibly high octane (120+ RON quite often) and therefore very detonation resistant even with leaner mixtures, but you’d have to be pretty damn rich to run your road car on it!
Only really viable for full race cars due to it’s highly toxic, corrosive and flammable nature, not to mention the fact you need about four times the amount of fuel than you need running normal petrol. It’s popular for full drag cars as it’s incredibly detonation resistant, very cold, often negating the need for an intercooler, and contains a lot of oxygen itself, further increasing power.
This fuel is becoming more common at the pumps in the UK, and while mostly advertised as a cheap and environmentally friendly fuel, the main attraction of it to performance tuning fans is it’s around 106 RON, and therefore very detonation resistant. Similar to methanol, it needs extra fuel to run at peak performance, but at much more road-friendly levels, and could become the performance fuel of choice once more filling stations decide to stock it.
At about half the price of normal fuel, it’s mostly used to save money, but it has performance potential too. LPG is incredibly resistant to detonation, so with a system optimised for power, LPG has the potential to be used as a great fuel on big power turbo/ supercharged engines.
Air fuel ratio in diesel engines
Diesel air fuel ratio issues are almost backwards to petrol, and it’s actually overly rich mixtures that cause problems: lean is good for the health of a diesel engine, if not for the amount of power it produces. A diesel engine runs incredibly lean compared to a petrol engine regardless, and richer mixtures are actually the things that increase heat in the engine and lead to components failing.
Detonation is much less of an issue with diesels, in fact the clattery noise in diesels is closely related to this. But, it isn’t damaging, apart from in certain situations where it’s incredibly severe, and unfortunately this is usually related, just like on petrol engines, to major tuning errors!